1. Field of the Invention
This invention relates to a polyethylene composite film suitable for packaging, which is superior in moisture-proofness, flexocracking resistance, tearability and heat sealability.
2. Description of the Prior Art
Up to the present time, as conventional packaging films have been known three layer structures each consisting of a surface layer of a heat resisting material such as paper, polyethylene terephthalate film (PETP), biaxially oriented polypropylene film (OPP), cellophane, oriented polyamide film (OPA) or the like, an intermediate layer of a gas barrier material such as saponified ethylene-vinyl acetate copolymer film (saponified EVA), aluminum foil (Al) or the like, and a sealant layer. However, these surface layers meet with the problems that the flexocracking resistance, moistureproofness and tearability are not sufficient to make up the performance of the intermediate layer and the surface layers need provision of sealant layers in uses, for example, gusset bags in which surface layer materials must have sealability each other, thus resulting in increase of the production cost, since the surface layers have no sealability.
On the other hand, a polyethylene film excellent in moisture proofness, clarity and tearability has been proposed which is crosslinked and oriented in such a manner that the degree of crosslinking is specified in the thickness direction (Japanese Laid-Open Publication Nos. 174321/1984 and 174322/1984). However, this crosslinked and oriented polyethylene film has excellent properties as a packaging film, but the heat sealability of this film itself or a laminated film using this film as a sealant is not always sufficient, in particular, in the case of making a heat-sealed bag by contacting the film with a heat sealing bar at a high temperature at a high speed.
Furthermore, a polyethylene composite film has also been proposed which consists of a crosslinked and oriented polyethylene film excellent in moistureproofness and clarity, having a specified degree of crosslinking in the thickness direction of the film, and a sealant layer of a low density polyethylene (Japanese Patent Laid-Open Publication No. 171149/1985). However, this polyethylene composite film has also a disadvantage in the heat sealability of the films one another.
As a film with a good tearability, cellophane or longitudinally or laterally monoaxially oriented polyethylene film (MOPE) has been known. However, the cellophane surely has a good self tearability property, but the straight line tearability is not good, and the monoaxially oriented polyethylene film has the disadvantage that the straight line tearability is good, but a clear tearability is hardly obtained because of tendency of occurrence of fibrils during cutting.
A standing pouch using a packaging film, of the conventional packagings, needs the straight line tearability. To this end, for example, there have been proposed laminated films with lamination structures of polyester film (PET film)/aluminum foil (Al foil)/MOPE film/non-oriented polypropylene film (CPP film) and PET film biaxially oriented nylon film (O-Ny film)/MOPE film/low density polyethylene film (LDPE film). However, these laminated films have also disadvantage of occurrence of fibrils and accordingly, it has eagerly been desired to develop a packaging film with more excellent linear and clear tearability.